The present invention relates generally to motion picture film projectors of the type including sprocketless film transport means for incrementally advancing each frame of the film past a projection gate, and more particularly to an improved buffer interposed between a supply reel for the film and the film transport means to insure the smooth advancement of the film through the projection gate.
Various types of buffers have been known in the prior art, since it has long been recognized that it is necessary to absorb the irregularities in the movement of the film created by the incremental step-wise movement thereof through the film gate when the film is pulled therethrough by the film transport means, such as a claw engaging the sprocket holes of the film, and a continuously rotating supply reel. For this purpose, the self-elasticity of the film has been utilized by looping the film around a stationary or swingable roller disposed between the supply reel and the projection gate. Buffers have also been provided wherein a swingable roller is spring loaded into a loop forming position. It has been found that such buffers operate satisfactorily as long as the film offers a relatively constant resistance to forward movement under the action of the film advancement mechanism. Accordingly, such conventional buffers are fully capable of compensating for the fact that the supply reel rotates continuously, whereas the film transport mechanism advances the film incrementally through the projection gate. In this regard, the buffer repeatedly swings between two positions to decrease and increase the loop size, and hence the film path distance, the first as a result of increasing tension wherein the film is pulled by the transport means, and the second as a result of the elasticity of the film as the supply reel rotates by inertia when the transport means returns to advance the next frame.
However, the aforementioned conventional buffers do not operate satisfactorily when the tension of the film fluctuates for reasons other than the fact that the film transport means draws the film intermittently off a continuously rotating supply reel. This can occur if the supply reel carries a large roll of film, or is out of round, or rotates with a certain amount of radial and/or axial play. Such unpredictable random fluctuations of the film tension often result in the generation of noise and in jumping or skipping of projected images because the frames are not properly aligned with the gate during projection. In the case of the spring loaded swingable buffers, it has been found that under circumstances of excessive tension the resonance of the spring contributes to the irregularities in the film projection, rather than compensating for the same.
To overcome this deficiency it has been proposed in U.S. Pat. No. 2,838,304 to Berkenhoff to provide means whereby the roll on a swing arm type buffer is placed in a position to be rotatably driven by a drive belt from the projection motor in a film advancing direction in those circumstances when the film on the reel offers great resistance to movement. However, such device tends to overcompensate for excessive film tension by advancing more film from the supply reel than can be immediately taken up by the transport means.
The problem of spring resonance has been dealt with in U.S. Pat. No. 3,942,883 to Osborne et al by utilizing a dash pot in operative engagement with the spring loaded swing arm. Unfortunately, such a device also includes a supplemental torque motor responsive to film tension and accordingly is extremely costly and complex.
Another complex film damping device, which utilizes spring loaded pivotally movable legs over which the film is looped, is shown in U.S. Pat. No. 3,977,775 to Riedel.